The field of the present invention is that of aircraft landing gears and more particularly that of systems for controlling the front landing gear thereof, in terms of retraction and in terms of steering.
Aircraft, particularly passenger airplanes, having a landing gear generally made up of a main landing gear consisting of two sets of wheels each situated under one of the wings of the airplane or under the fuselage, and of a nose landing gear situated near the front end of the fuselage. This nose gear, in addition to supporting the weight of the airplane, has a function of steering when the airplane is on the ground taxiing. It is also retractable, like the main landing gear, so that it can be housed in the fuselage after takeoff thus avoiding aerodynamic friction forces, known as drag, which, when exerted on it, would increase the fuel consumption.
This landing gear is, in the conventional way, actuated by two actuating cylinder systems one of which has the function of retracting the landing gear into the fuselage after takeoff and the other of which has the function of steering the landing gear by rotating its wheels in one direction or the other about the vertical axis of the landing gear so as to steer the airplane when it is on the ground taxiing.
With reference to FIG. 1, a nose landing gear conventionally comprises a structural component or leg 1 which ends at its upper part in arms which form a pivot 2 about which the gear is retracted. The pivot 2 is mounted on a structural component of the airplane cell so as to bear the weight of the nose landing gear when the airplane is in flight and transmit the weight of the nose section of the airplane to the landing gear when the airplane is on the ground. This pivot allows the leg 1 to be retracted as the landing gear is raised, under the action of the retraction actuating cylinder or cylinders 12a and 12b. The leg is connected to the structure of the airplane, firstly by the retraction pivot 2, and secondly by a leg bracing strut, known as the main leg bracing strut 3, which has an articulation 4 to allow it to fold as the landing gear is raised. This main leg bracing strut has the function of preventing unwanted folding of the landing gear when external forces are applied to the landing gear, for example when the wheels impact with the ground upon landing. A secondary leg bracing strut 5 deploys when the landing gear comes down and prevents the main leg bracing strut 3 from folding. It can itself be folded so that it can be retracted under the action of an actuating cylinder that is operated when the landing gear is raised, and thus allow the main leg bracing strut 3 to fold and the leg 1 to be raised up into the fuselage.
The leg 1 conventionally takes the form of a hollow cylinder in which a rotary tube 6 is placed. The rotary tube is held longitudinally in place in the leg by means known to the person skilled in the art and is capable of rotating to allow the steering instruction given by the pilot to be applied during taxiing. When the landing gear is retracted after takeoff, the rotary tube is first of all returned, by a system which may be internal or external to the leg, to a rest position which corresponds to the wheels of the nose gear lying along the axis of the airplane. In general, a mechanical system incorporated into the landing gear confirms the alignment of the wheel upon takeoff and therefore also while the shock absorber is extending.
The rotary tube is itself hollow and allows a sliding rod 7 to move within it, which sliding rod projects toward the bottom of the rotary tube and carries the axle for the nose gear wheels. The sliding rod 7 is fixed to the rotary tube by a shock absorber so as to move inside the leg according to the vertical forces applied to the wheels and so as to return to a central position which, at rest, corresponds to equilibrium between the weight borne by the nose gear and the reaction force of said spring. The sliding rod 7 and the rotary tube 6 are connected by a collection of components known as a torque link 8 which transmits to the sliding rod the orientation given to the rotary tube and therefore provides the steering command for the wheels of the airplane. The torque link assembly 8 is made up of two articulated link rods; a first link rod is attached, at one of its ends, to a horizontal pivot connected to the rod and at the other end to one of the ends of the second link rod; the other end of the second link rod is attached to a horizontal pivot connected to the rotary tube 6. The common ends of the two link rods are fixed to one another about a common horizontal pivot.
Because of the shock absorber installed inside the rotary tube, the sliding rod 7 is capable of absorbing any shocks applied to the wheels, by moving vertically.
As the sliding rod 7 moves the torque link assembly 8 deforms, remaining in a plane that is radial to the leg, and thus forces the wheels to remain in a fixed direction in relation to the rotary tube 6. The wheels are thus steered by instructing the rotary tube 6 to turn.
Numerous systems have been devized for controlling the nose gear of an aircraft, whether in terms of retracting it into a well provided for this purpose in the aircraft fuselage, or for steering when taxiing on the ground. As indicated previously, they generally consist of two actuating cylinder systems, one of which acts on the leg 1 of the nose gear in order to retract it after take off and deploy it prior to landing and the other of which acts on the rotary tube 6, in order to steer the wheels.
The actuating cylinder or cylinders concerned with retraction are fixed to the structure of the aircraft and work either in tension or, and for preference, in compression, on a fulcrum which may be situated on the leg either above or below the retraction pivot 2, depending on the geometric configuration adopted. The steering actuating cylinders, of which for preference there are two, act on the rotary tube at a point generally situated below the leg 1.
Nose gear steering systems that allow the rotary tube 6 to be pivoted so that the wheel effects a quarter of a turn and fits more easily into the fuselage when the landing gear is retracted are also known (patent applications FR 1473951 and DE 941109). The devices which generate this rotation are generally attached to fulcrums positioned on the rotary tube in the upper part of the landing gear but are engineered only to be able to pivot the rotary tube, equipped with the sliding part and with the wheels, in the leg. They are not able to withstand the forces applied to the leg when the landing gear is being retracted.
The configurations described hereinabove have the disadvantage that a number of nose gear equipments (steering actuating cylinders, supply lines and points of attachment for these actuating cylinders) are situated in a low position on the landing gear and are therefore placed outside the fuselage when the landing gear is down. These equipments are therefore positioned in the wind surrounding the airplane, and this generates aerodynamic noise and drag which is detrimental to fuel consumption. It is important, given the changes to the regulations, to reduce as far as possible the causes of noise generation and there is also a constant drive toward reducing as far as possible the overall drag of the airplane. To do so it is preferable for the equipments that control the nose gear to be positioned as high up as possible inside the fuselage. That means reducing the number and/or the volume and/or the mass of these equipments.